Conventional mechanisms comprise a metal tube coaxial with the rod and mounted to slide in electrical contact thereon, and having an end that plugs onto a conductive plug of the switch so as to short-circuit the resistor. The end of the tube that plugs onto the conductive plug is formed of a thimble of flexible fingers. A spring is disposed between a first shoulder provided on the rod and a second shoulder provided in the tube so as to be compressed during a first stage of switch opening, and so as to exert a return force on the tube to displace it in said direction in order to separate it form the plug during a second stage of switch opening subsequent to the first stage. When the switch is closed, the thimble of fingers of the metal tube is plugged onto the conductive plug which is normally connected to a terminal of the switch. During opening, it is important that the thimble of fingers on the metal tube does not come unplugged from the conductive plug before the contacts of the switch are fully separated. That is why the mechanism for inserting the resistor is designed so that the force form the compressed spring always remains lower than the tube-unplugging force, the tube being unplugged by the rod of the semi-moving contact coming into abutment in the axial direction against the metal tube.
In addition to the fact that the force at the flexible fingers of the metal tube is difficult to control, wear on the fingers over time gives rise to a loss of reliability of the mechanism for inserting the closure resistor.